The present invention relates to a single-lens reflex camera having a focusing detection sensor capable of calibrating, and a viewfinder.
In many of the prior art single-lens reflex cameras, a focusing detection sensor, which is used to detect a focal point on an image of an object, is usually located below a main mirror within a camera body. Thus, the single-lens reflex camera having the focal point sensor in a position of or behind a focusing plate is not generally available yet.
Japanese Patent Laid-Open No. 2000-75344, for instance, discloses the aforementioned type of the single-lens reflex camera having a focusing detection sensor located below a main mirror within its body.
However, in such a prior art single-lens reflex camera that is configured to have the focusing detection sensor located below the main mirror within the camera body, a space must be reserved for the sensor below the main mirror, and this disadvantageously disturbs an attempt to downsize the camera body.
If the focusing detection sensor is located in a position of or behind the focusing plate within a replaceable viewfinder, accuracies in terms of an attachment of the replaceable viewfinder and of a location of the focusing detection sensor must be considerably enhanced to minimize a precision degradation in detecting a focal point on an object image which is resulted from varied combinations of compatible but independent units of the replaceable finder and camera body.
Furthermore, since such a replaceable viewfinder is usually of relatively large dimensions, an optical length from the camera body to the focusing detection sensor is liable to alter due to thermal expansion of optical components and metal parts used therein, and thus, it is considerably hard to retain a satisfactory level of accuracy in detecting the focal point on the object image.
Accordingly, it is an object of the present invention to provide a single-lens reflex camera used in combination with a replaceable viewfinder which has a focal point detecting sensor built therein but is nevertheless capable of detecting a focal point on an image of an object at high accuracy, and specifically, an inventors"" attempt is to devise an arrangement of the camera and viewfinder that is capable of accurately detecting a deviation of an imaging field (image plane of the object) of a photographic lens from an optically equivalent plane to film (i.e., detecting a defocusing length or a focal shift).
It is another object of the present invention to provide a single-lens reflex camera used in combination with a replaceable viewfinder which is capable of keeping a detection speed of a focal shift sufficiently high and eliminating inconvenience of a camera user during the calibration for the purpose of detecting a focal shift at high accuracy.
In accordance with the present invention which is made to overcome the aforementioned disadvantages in the prior art, a single-lens reflex camera is comprised of a camera body having a focusing plate with a reference mark in an optically equivalent position to film, a viewfinder replaceably attached to the camera body, means positioned in the viewfinder for detecting a focal point, means positioned in the viewfinder for determining deviations for the reference mark and the focal point, respectively, calibration means receiving the output from the deviation determining means for correcting data on the focal point, and means enabling a user to activate the calibration means.
In one aspect of the present invention, a sensor is used to detect a focal point on an image of the reference mark provided in the focusing class within the camera body. The calibration means receives a detection result from the focusing detection sensor to compute a variation in an optical length from the focusing plate to the focusing detection sensor. The means serving as a trigger enables the user to activate the calibration means so that the variation in an optical length from the focusing plate to the sensor is calibrated. A deviation of the focal point on the object image, which is detected by the focusing detection sensor, is corrected based upon the calibration result. In this way, an error resulted from the varied optical length is corrected, and thus, an enhanced accuracy in detecting a defocusing length can be attained.
Preferably, the calibration means includes a memory to store the calibration result on the optical length.
In one aspect of the present invention, since the calibration result can be stored once it is obtained, there is no need of repeating the calibration for each photo shooting.
The calibration means preferably includes an illumination means placed in the camera body or the viewfinder for irradiating the reference mark, and additionally, it is desirable to use the calibration means under a condition where light entering the camera body through a photographic lens is cut off.
In one aspect of the present invention, the calibration is carried out without influence of the light entering the photographic lens, and hence, a calibration accuracy can be improved.
The calibration preferably includes a sensing means for determining if the light entering the camera body through the photographic lens is satisfactorily cut off.
Advantageously, an additional means is included to give an alert or a display to a user and let him or her to know that the calibration means is in use.
Also, in accordance with the present invention, a viewfinder is used with a single-lens reflex camera and replaceably attached to a camera body having a focusing plate therein. The viewfinder is capable of detecting a reference mark which is formed in the focusing plate and positioned in an optically equivalent position to film, and it comprises means for determining a focal point of the finder, means for determining deviations for the reference mark and the focal point, respectively, calibration means receiving the output from the deviation determining means for correcting data on the focal point, and means enabling a user to activate the calibration means.